A series of enzymological, stereochemical and mechanistic studies are proposed on reactions in hexose metabolism, which lead to the reductive replacement of hydroxyl groups by hydrogen and to the epimerization of chiral centers in the carbohydrate chain. These studies will deal with the GDP-mannose oxidoreductase reaction involved in fucose biosynthesis, with the CDP-4-keto-6-deoxyglucose 3-reductase system from Salmonella, with the formation of 2,6-dideoxyhexoses in several streptomyces, with the epimerization of GDP-mannose to GDP-L-galactose (and GDP-L-fucose) by plant enzymes, and with the epimerization of D mannuronic to L guluronic acid residues in the course of alginic acid formation.